.ACT
Adobe Color Table - Used to store 256-color palette data for RAW textures
.TRK
Truck File - Provides all information pertaining to how a truck behaves in-game. 
Add data from official .trk document.
.OPA
Opacity Texture - Used to provide information on what parts of the truck can be seen through, and what areas should have paint applied.
.RAW
RAW Pixel Texture - 8-bit texture data, only contains pixel values. It does not include sizing, palette data, or any attributes. Texture size is used to infer the correct sizing. NOTE: Some textures in game are hard coded to be non-square, or store multiple color channels. (ex. Loading screen/UI elements)
.SMF
Simple Model File. Plain text model format specific to the 4×4 Evolution series.
.SMB
Simple Model Binary. Binary version of above, used only on console version to reduce load times. Not supported on PC version.
.TIF
Used to store textures for 4x4 Evolution 2. NOTE: it uses a non-standard TIF format that includes a 4th color channel to provide opacity texture data on trucks.